There has been known a super-twisted element for a method of obtaining a high density dot matrix display by increasing a twist angle of liquid crystal molecules between a pair of electrodes to thereby cause a sharp change of voltage-transmittance characteristics (T. J. Scheffer and J. Nehring, Appl. Phycs. Lett. 45(10)1021-1023(1984).
In the conventional method, however, the product .DELTA.n.multidot.d of the refractive index .DELTA.n of liquid crystal in a liquid crystal display element used and the thickness d of a liquid crystal layer was substantially in a range of 0.8 .mu.m to 1.2 .mu.m (Japanese Unexamined Patent Publication No. 10720/1985 which is referred to as conventional technique 1). According to the conventional technique, an excellent contrast could be obtained only by a specified combination of colors such as yellowish green and dark blue, bluish purple and pale yellow and so on.
Thus, in the conventional technique using STN liquid crystal display element, a monochrome display could not be effected.
In order to improve the conventional technique, there was proposed a liquid crystal display apparatus capable of displaying a monochrome display and having a high contrast ratio wherein two liquid crystal cells of different helical structures are placed one on another; a voltage is applied to either cell and the other is merely used as an optically compensating plate (Report of Television Association 11 (27), p. 79 (1987) by Okumura et al.).
Also, there was proposed a method of providing a monochrome display by arranging a birefringent plate between the liquid crystal layer and a polarizing plate. Conventionally, a color liquid crystal display apparatus used for OA machines such as personal computers comprised the above-mentioned liquid crystal display element capable of effecting a monochrome display and color filters.
However, the color filters are expensive and have extremely low efficiency of utilization of light since a display is effected with three picture elements of red, blue and green. For instance, three picture elements of red, blue and green are used for displaying white, and even when the three picture elements are turned on, the brightness is 1/3 and accordingly, a bright display can not be obtained. Several techniques have been proposed for color display apparatuses providing a bright display without having color filters. For instance, an electrically controlled birefringence (ECB) effect type liquid crystal display apparatus is known. In this apparatus, when gradation voltages (e.g., voltages for 8 gradations) are applied to a pixel, the orientation of liquid crystal molecules is changed depending on gradation voltages applied whereby .DELTA.n.multidot.d of the liquid crystal cells is changed. And various colors caused by the effect of the birefringence in the liquid crystal cell are used.
In such ECB effect type liquid crystal display apparatus, however, since liquid crystal does not have a twisted structure, a state of liquid crystal to be changed depending on an applied voltage was small, and a display by multiplex driving could not be obtained.
Japanese Unexamined Patent Publication No. 118516/1990 (conventional technique 2) discloses that various colors can be displayed by changing a voltage applied to a liquid crystal cell including twisted liquid crystal molecules. In this conventional technique 2, however, there is a problem that colors which can be developed are yellow, red, purple, bluish purple, bluish green and green, and a display of achromatic color such as black or white is impossible, because it has been known that the visibility in usually used displays is considerably reduced if a display of black or white is not used.
In a display, the background portion generally occupies a broader surface area in comparison with a data portion to be observed. When a color of, for instance, yellow or green is used for that portion as a background color, it is difficult to obtain a non-simulative color. Accordingly, in a display of graphs or the like, achromatic color of black or white is often chosen as the background color.
As the basic of display, there is an expression with a line of black color on a white ground such as a letter or letters in black on a white paper, and such type of display is usually used. It is preferable to provide a display in blue, green and/or red in addition to a white/black display. Accordingly, a display apparatus which can not provide a white/black display lacks visibility.
The conventional technique 2 discloses that two layered structure using a compensation cell can provide a monochrome display. In this conventional technique, however, color development is achieved by applying a voltage to the compensation cell so that the compensation cell does not function in an optical sense. Accordingly, in the display apparatus to be operated by multiplex driving, it is impossible to mix a color of blue or green with white or black.
Japanese Unexamined Patent Publication No. 183220/1990 (conventional technique 3) discloses that pixels are also formed in a compensation cell to provide a display, and when each display of the two layered liquid crystal cells is combined with each other and multiplex driving is conducted, a color of blue or green can be provided along with black or white. However, each of the pixels in the two cells has to be formed in a one to one relation. In this case, the manufacture of the liquid crystal display apparatus is difficult. Further, when the apparatus is watched from an oblique direction, mixing of color is observed due to parallax.
The conventional technique is insufficient to provide a display of quality of practically usable. Further, the liquid crystal display apparatus having a double layered structure increases the weight; it is difficult to control the gap in the liquid crystal cells, and yield of manufacture is further decreased.
Japanese Unexamined Patent Publication No. 175125/1994 (conventional technique 4) discloses that an improvement of color can be obtained by using a birefringent plate. However, this publication does not disclose a display of achromatic color (white or black).
Japanese Unexamined Patent Publication No. 301006/1994 (conventional technique 5) discloses in some embodiments that it is possible to display colors of blue, green, white and red. However, this conventional technique is so adapted that a display of blue color is provided when an applied voltage is low and a white color is developed when the applied voltage is increased. Accordingly, when lattice-like matrix driving is effected and if the color of spaces between driving electrodes are blue, a generally blue display is provided even though the color of pixels is white, and white having good color purity can not be developed. Namely, it is preferable that the spaces are substantially of achromatic color unless the voltage is applied. Further, when an achromatic color is to be presented by applying a voltage of intermediate tone, a slight change of voltage causes a change of color in a display since liquid crystal molecules to which an intermediate voltage is applied show a sudden change with a slight change of voltage. Accordingly, a beautiful display of achromatic color can not be obtained.
The same situation is applicable to a case that colors which are developed by applying intermediate voltages are used in the whole area of picture display. Generally, an achromatic color is used for the background color. In this case, the area of achromatic color occupies a large surface area. When the color occupying such a large surface area is deteriorated, the quality of display is considerably reduced. Accordingly, it is desirable to avoid the development of the achromatic color at an intermediate voltage in order to obtain a uniform color.
In consideration of the above-mentioned problems, it is preferable that a display of the achromatic color can be obtained when no voltage is applied or an OFF waveform (non-selection waveform) is formed in multiplex driving.
In Example 5 of the conventional technique 5, there is description that a display of white, blue and green is possible. However, it also discloses that an applied voltage for developing white is 0.2 V or less, an applied voltage for developing blue is 1.3 V to 2.2 V and an applied voltage for developing green is 3.0 V or more. In the conventional technique 5, it is apparently difficult to effect multiplex driving (time sharing driving). The driving voltages can be used only for a specified purpose of use.
Embodiment 6 in Japanese Unexamined Patent Publication No. 301026/1994 (conventional technique 6) describes that a white display can be obtained with 0.9 V or 1.6 V or less. However, a large duty ratio can not be utilized for multiplex driving when a display of green, red or blue is to be presented.
An embodiment in Japanese Unexamined Patent Publication No. 337397/1994 (conventional technique 7) describes that a white display is obtained when an OFF waveform is formed. However, the conventional technique 7 can not provide a display of red. In the above-mentioned conventional techniques, polarizing plates are provided at both sides of the cell or cells. Accordingly, the transmittance of light is low. Further, when the conventional techniques are used as a reflection type mode, a light quantity is further reduced because there is a loss at the reflection layer, and therefore, a display becomes further dark.
In an embodiment of Japanese Unexamined Patent Publication No. 308483/1994 (conventional technique 8), a color display is provided with use of a single polarizing plate. Conventional technique 8 has the constitution that .DELTA.n.multidot.d value of the liquid crystal cell for color controlling (compensation layer ) and .DELTA.n.multidot.d value of the liquid crystal cell for display are substantially same and the direction of their twist angle is reversed.
And the angle formed by the two orientation directions at the opposite surface plane of two liquid crystal cells (display,compensation) which are to be arranged to be opposed each other is set to be about 90.degree.. Further it is described that the polarizing axis of the polarizing plate is favorable to be set at a crossing angle of 45.+-.5.degree.. In that embodiment, there is description that the white color is developed when no voltage is applied, and when an applied voltage is gradually increased, a color change of white.fwdarw.yellow.fwdarw.blue.fwdarw.yellow.fwdarw.blue.fwdarw.green is effected. However, it discloses that when the applied voltage is low (i.e. when a voltage for effecting a color change of white.fwdarw.yellow or white.fwdarw.blue is applied) color development having good color purity can not be obtained, and the color development having good purity can be obtained by increasing the voltage.
Further, in the conventional technique 8, color development of red can not be obtained. Further, the conventional technique is not suitable for multiplex driving since color development having good purity can knot be obtained when an applied voltage is low.
Japanese Unexamined Patent Publication No. 5457/1995 (conventional technique 9) discloses that a twist birefringent plate is used as a compensation cell. However, the construction of the conventional technique 9 is basically the same as that of the conventional technique 8 and it still has a problem of color development.
There was made a PCT application No. PCT/JP96/00101 by the same applicant whose subject is a reflection type color liquid crystal apparatus constitutes one birefringent plate, two polarizing plate, a liquid crystal layer and so on without using color filters.
In this application a RC-LCD which has only one polarizing plate is provided.
It is an object of the present invention to provide a color liquid crystal display apparatus which allows multiplex driving; exhibits a bright white display by applying a non-selection waveform, and develops a color of blue or green or red without using color filters when a selection waveform or an intermediate voltage between the selection waveform and the non-selection waveform is applied.
In other words, the object of the present invention is to provide a reflection type color liquid crystal display apparatus capable of providing a very bright display of substantially achromatic color when no voltage is applied or an applied voltage is low, and capable of realizing a color display by applying a voltage.
Hereinafter in this description the sign plus (+) means for the direction of clockwise and the sign minus (-) means for the direction of counterclockwise. This feature is shown in FIG. 1 and so on. In this application, a research was conducted by a computer simulation in viewpoint of a relation between a optical characteristic and the constitutional element of a reflection type color liquid crystal display apparatus such as .DELTA.n.sub.1 .multidot.d.sub.1, .DELTA.n.sub.2 .multidot.d.sub.2, crossing angle conditions by .theta..sub.1 and so on. Thus the constitutional elements for desirable color development has been substantially known.
The inventors of this application actually manufactured some embodiments of the liquid crystal display apparatus of the present invention, for instance, examples No. B8, B9, B10 mentioned later and they confirmed the achievement of the present invention by using a simulator of liquid crystal device, which is widely used for calculating the optical characteristics of liquid crystal and which is called the 4.times.4 matrix method by Berreman. In this method of calculation, first, a state of orientation of liquid crystal to which a voltage is applied is obtained by calculation. Then, optical members such as liquid crystal, compensation films, polarizing plates and so on are divided into a plurality of layers of appropriate thickness, and the local propagation matrix is calculated for each of the divided layers.
Then, the values of the local propagation matrix of each of the layers are multiplied to obtain the propagation matrix of the all optical members. Thereafter, the reflection light and the transmitting light of incident light are calculated by using the propagation matrix.
The transmittance and the reflectance of lights of various wavelengths can be calculated by using the 4.times.4 matrix method by Berreman. By using this method, the luminous transmittance and the luminous reflectance could be calculated, and x values and y values of chromaticity coordinates for the color liquid crystal display apparatus of the present invention could be calculated quickly and accurately.
The 4.times.4 matrix method by Berreman is known as a technique capable of beautifully reproducing experimental results by numerical calculations. The inventors compared the experimental results of examples of the present invention with values obtained by numerical calculations, and confirmed that the experimental results and the values obtained by numerical calculations substantially agreed in the range of effective precision.
An actually used LCD has an ITO, a glass substrate or a spacer for a gap control which has a finite transmittance and wavelength characteristics. In consideration of influence by these elements, the transmittance vs applied voltage characteristics obtained by calculations well reproduced the experimental results.
Further, a result of calculation of the chromaticity substantially corresponds to the experimental results. Accordingly, calculations by the Berreman's 4.times.4 matrix method can be used for actual experiments.
In the basic construction of the present invention, a single birefringent plate in which there are optical anisotropic axes at its both surfaces and one of the optical anisotropic axes is in a relation of rotation with respect to the other optical anisotropic axis, and a liquid crystal layer in which the orientations of liquid crystal molecules at its both surfaces show a twist angle, are interposed between a single polarizing plate having the absorbing axis and a reflection layer, wherein the positions of the birefringent plate and the liquid crystal layer are interchangeable, and light is passed from the polarizing plate through the birefringent plate and the liquid crystal layer or through the liquid crystal layer and the birefringent plate, and reflected at the reflection layer to be propagated in the opposite direction and emitting through the polarizing plate.
In this case, effective driving voltages of three values or more are applied to the liquid crystal layer whereby a color display including an achromatic color can be obtained.
Claim 1 concerns a reflection type liquid crystal display apparatus comprising:
a polarizing plate,
a birefringent plate having a twist angle,
a liquid crystal layer of a nematic liquid crystal having positive dielectric anisotropy and including a chiral material, which is interposed between two substrates disposed to each other, each provided with an electrode, and
a reflection layer; wherein
said liquid crystal layer has
a twist angle .theta..sub.1 from the first orientation direction at its first plane of said liquid crystal layer to the second orientation direction at its second plane of the same; PA1 said birefringent plate has a twist angle .theta..sub.2 from the first slow axis of optical anisotropic axes in a plane at the side for the polarizing plate to the second slow axis of optical anisotropic axes in opposite plane in a direction from the first slow axis to the second slow axis, and PA1 the product of .DELTA.n.sub.1 .multidot.d.sub.1 of the refractive index anisotropy .DELTA.n.sub.1 of the liquid crystal in the liquid crystal layer and the thickness d.sub.1 of the liquid crystal layer is 0.30 to 2.00 .mu.m; PA1 the product of .DELTA.n.sub.2 .multidot.d.sub.2 of the refractive index anisotropy .DELTA.n.sub.2 of the birefringent plate and the thickness d.sub.2 of the birefringent plate is 0.30 to 2.00 an angle .theta..sub.3 is formed from said first orientation direction of the liquid crystal layer to the second slow axis of said birefringent plate; PA1 an angle .theta..sub.4 is formed from said first slow axis of said birefringent plate to the absorbing axis of said polarizing plate; PA1 {.theta..sub.1 is -160.degree. to -300.degree., PA1 .theta..sub.2 is +160.degree. to +300.degree., PA1 .theta..sub.3 is (+90.degree.+-10.degree. to +40.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-30.degree. to +25.degree.! or +135.degree.+-30.degree. to +25.degree.!)}; or PA1 {.theta..sub.1 is +160.degree. to +300.degree., PA1 .theta..sub.2 is -160.degree. to -300.degree., PA1 .theta..sub.3 is (-90.degree.++10.degree. to -40.degree.!) and PA1 .theta..sub.4 is (-45.degree.++30.degree. to -25.degree.! or -135.degree.++30.degree. to -25.degree.!)}; PA1 .vertline..theta..sub.1 .vertline. is 230.degree. to 250.degree., PA1 .vertline..theta..sub.2 .vertline. is 230.degree. to 250.degree., PA1 0.8.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.1.5 and PA1 .DELTA.n.sub.1 .multidot.d.sub.1 -0.15.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 +0.15. PA1 {.theta..sub.1 is -230.degree. to -250.degree., PA1 .theta..sub.2 is +230.degree. to +250.degree., PA1 .theta..sub.3 is (+90.degree.+-10.degree. to +10.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-20.degree. to -10.degree.! or +135.degree.+-20.degree. to -10.degree.!)}; or PA1 {.theta..sub.1 is +230.degree. to +250.degree., PA1 .theta..sub.2 is -230.degree. to -250.degree., PA1 .theta..sub.3 is (-90.degree.++10.degree. to -10.degree.!) and PA1 .theta..sub.4 is (-45.degree.++20.degree. to +10.degree.! or -135.degree.++20.degree. to +10.degree.!)}; PA1 1.25.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.1.35 and PA1 1.25.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq.1.35. PA1 {.sbsp.-230.degree. to -250.degree., PA1 .theta..sub.2 is +230.degree. to +250.degree., PA1 .theta..sub.3 is (+90.degree.++10.degree. to +30.degree.!) and PA1 .theta..sub.4 is (+45.degree.++5.degree. to +25.degree.! or +135.degree.++5.degree. to +25.degree.!)}; or PA1 {.theta..sub.1 is +230.degree. to +250.degree., PA1 .theta..sub.2 is -230.degree. to -250.degree., PA1 .theta..sub.3 is (-90.degree.+-10.degree. to -30.degree.!) and PA1 .theta..sub.4 is (-45.degree.+-5.degree. to -25.degree.! or -135.degree.+-5.degree. to -25.degree.!)}; PA1 0.80.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.0.90 and PA1 0.80.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq.0.90. PA1 {.theta..sub.1 is -230.degree. to -250.degree., PA1 .theta..sub.2 is +230.degree. to +250.degree., PA1 .theta..sub.3 is (+90.degree.+-10.degree. to +10.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-30.degree. to -10.degree.! or +135+-30.degree. to -10.degree.!)}; or PA1 {.theta..sub.1 is +230.degree. to +250.degree., PA1 .theta..sub.2 is -230.degree. to -250.degree., PA1 .theta..sub.3 is (-90.degree.++10.degree. to -10.degree.!) and PA1 .theta..sub.4 is (-45.degree.++30.degree. to +10.degree.! or -135.degree.++30.degree. to +10.degree.!)}; PA1 1.2.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.1.30 and PA1 1.30.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq.1.40. PA1 {.theta..sub.1 is -230.degree. to -250.degree., PA1 .theta..sub.2 is +230.degree. to +250.degree., PA1 .theta..sub.3 is (+90.degree.+-10.degree. to +40.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-30.degree. to +25.degree.! or +135.degree.+-30.degree. to +25.degree.!)}; or PA1 {.theta..sub.1 is +230.degree. to +250.degree., PA1 .theta..sub.2 is -230.degree. to -250.degree., PA1 .theta..sub.3 is (-90.degree.++10.degree. to -40.degree.!) and PA1 .theta..sub.4 is (-45.degree.++30.degree. to -25.degree.! or -135.degree.+ +30.degree. to -25.degree.!)}; PA1 1.20.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.1.50 and PA1 0.70.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 &lt;0.90. PA1 {.theta..sub.1 is -170.degree. to -190.degree., PA1 .theta..sub.2 is +170.degree. to +190.degree., PA1 .theta..sub.3 is (+90.degree.+-10.degree. to +40.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-30.degree. to +25.degree.! or +135.degree.+-30.degree. to +25.degree.!)}; or PA1 {.theta..sub.1 is +170.degree. to +190.degree., PA1 .theta..sub.2 is -170.degree. to -190 , PA1 .theta..sub.3 is (-90.degree.++10.degree. to -40.degree.!) and PA1 .theta..sub.4 is (-45.degree.++30.degree. to -25.degree.! or -135.degree.++30.degree. to -25!)}; PA1 0.30.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.2.00, and PA1 0.30.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq.2.00. PA1 {.theta..sub.1 is -170.degree. to -190.degree., PA1 .theta..sub.2 is +170.degree. to +190.degree., PA1 .theta..sub.3 is (+90.degree.+-10.degree. to +40.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-30.degree. to +25.degree. ! or +135.degree.+-30.degree. to +25.degree.!)}; or PA1 {.theta..sub.1 is +170.degree. to +190.degree., PA1 .theta..sub.2 is -170.degree. to -190.degree., PA1 .theta..sub.3 is (-90.degree.++10.degree. to -40.degree.!) and PA1 .theta..sub.4 is (-45.degree.++30.degree. to -25.degree.! or -135.degree.++30.degree. to -25.degree.!)}; PA1 0.90.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.1.30 and PA1 .DELTA.n.sub.1 .multidot.d.sub.1 -0.15.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 +0.15. PA1 {.theta..sub.1 is -170.degree. to -190.degree., PA1 .theta..sub.2 is +170.degree. to +190.degree., PA1 .theta..sub.3 is (+90.degree.++10.degree. to +30.degree.!) and PA1 .theta..sub.4 is (+45.degree.+-20.degree. to +20.degree.! or +135.degree.+-20.degree. to +20.degree.!)}; or PA1 {.theta..sub.1 is +170.degree. to +190.degree., PA1 .theta..sub.2 is -170.degree. to -190.degree., PA1 .theta..sub.3 is (-90.degree.+-10.degree. to -30.degree.!) and PA1 .theta..sub.4 is (-45.degree.++20.degree. to -20.degree.! or -135.degree.++20.degree. to -20.degree.!)}; PA1 0.75.ltoreq..DELTA.n.sub.1 .multidot.d.sub.1 .ltoreq.1.05 and PA1 0.75.ltoreq..DELTA.n.sub.2 .multidot.d.sub.2 .ltoreq.1.05. PA1 a twist angle .theta..sub.1 from the first orientation direction at its first plane of said liquid crystal layer to the second orientation direction at its second plane of the same; and PA1 the product of .DELTA.n.sub.1 .multidot.d.sub.1 of the refractive index anisotropy .DELTA.n.sub.1 of the liquid crystal in the liquid crystal layer and the thickness d.sub.1 of the liquid crystal layer is 0.30 to 2.00 .mu.m; PA1 the product of .DELTA.n.sub.2 .multidot.d.sub.2 of the refractive index anisotropy .DELTA.n.sub.2 of the birefringent plate and the thickness d.sub.2 of the birefringent plate is 0.30 to 2.00 .mu.m; PA1 an angle .theta..sub.5 formed from said first orientation direction of the liquid crystal layer to the second slow axis of said birefringent plate; PA1 an angle .theta..sub.6 formed from said first orientation direction of the liquid crystal layer to the absorbing axis of said polarizing plate; PA1 (.theta..sub.1 is -160.degree. to -300.degree.), (.theta..sub.5 is +70.degree. to +120.degree.) and (.theta..sub.6 is +25.degree. to +80 .degree. or +115.degree. to +170.degree.!); or PA1 (.theta..sub.1 is +160.degree. to +300.degree., (.theta..sub.5 is -70.degree. to -120.degree.) and (.theta..sub.6 is -25.degree. to -80.degree. or -115.degree. to -170.degree.)!; and
the angles .theta..sub.1, .theta..sub.2, .theta..sub.3 and .theta..sub.4 in a clockwise (+) or in counterclockwise (-) direction have such a relation that
and voltage values of at least 3 values are selected to be applied to said liquid crystal layer.
Claim 2 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 3 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 4 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 5 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 6 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 7 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 8 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 9 concerns the reflection type color liquid crystal apparatus according to claim 1, wherein
Claim 10 concerns the reflection type color liquid crystal apparatus according to any claim of claims 1-9, wherein
a relationship (1) and (2) is satisfied EQU (10.degree.).sup.2 .ltoreq.(.vertline..theta..sub.3 .vertline.-90.degree.).sup.2 +(.vertline..theta..sub.4 .vertline.-45.degree..sup.2 ( 1) EQU (10.degree.).sup.2 .ltoreq.(.vertline..theta..sub.3 .vertline.-90.degree.).sup.2 +(.vertline..theta..sub.4 .vertline.-135.degree.).sup.2 ( 2).
Claim 11 concerns a reflection type liquid crystal display apparatus comprising:
a polarizing plate,
a birefringent plate having not a twist angle,
a liquid crystal layer of a nematic liquid crystal having positive dielectric anisotropy and including a chiral material, which is interposed between two substrates disposed to each other, each provided with an electrode, and
a reflection layer; wherein
said liquid crystal layer has
the angles .theta..sub.1, .theta..sub.5 and .theta..sub.6 in a clockwise (+) or in counterclockwise (-) direction have such a relation that
voltage values of at least 3 values are selected to be applied to said liquid crystal layer.
Claim 12 concerns the reflection type color liquid crystal apparatus according to any claim of claims 1-9, wherein
a relationship (3) and (4) is satisfied EQU (10.degree.).sup.2 .ltoreq.(.vertline..theta..sub.5 .vertline.-90.degree.).sup.2 +(.vertline..theta..sub.6 .vertline.-45.degree.).sup.2 ( 3) EQU (10.degree.).sup.2 .ltoreq.(.vertline..theta..sub.5 .vertline.-90.degree.).sup.2 +(.vertline..theta..sub.6 .vertline.-135.degree.).sup.2 ( 4).
In the present invention, a reflection layer is arranged at the side of the liquid crystal layer in a backside substrate in other embodiment.
In the present invention, a temperature compensated circuit is arranged in one module with display element in other embodiment.
In the present invention, an adjustment means is arranged in one module with display element in other embodiment.
There is provided a reflection type color liquid crystal display apparatus according to any one of the above-mentioned inventions, wherein an N number of frames are used in one period, and in the N frames, an M number of ON waveforms and an (N-M) number of OFF waveforms are produced in other embodiment.
There is provided a reflection type color liquid crystal display apparatus according to the above-mentioned inventions, wherein a white display is effected when M=O; a green display is effected when M=N, and a red display or a blue display is effected when M.noteq.O and M.noteq.N in other embodiment.
There is provided a reflection type color liquid crystal display apparatus according to the above-mentioned inventions, wherein M is determined with respect to N to produce a gradation voltage thereby effecting a color display in other embodiment.
There is provided a reflection type color liquid crystal display apparatus according to the above-mentioned inventions, wherein the reflection layer is used as electrodes for driving the liquid crystal layer.
In each of the above-mentioned inventions, the first liquid crystal layer functions as an active optical layer for driving display data. Basically, a liquid crystal cell is used wherein transparent electrodes and an aligning control layer (or orientation layer) are disposed at both sides. In a case that the reflection layer side and the electrode at the back are commonly used, a transparent electrode is only used in the liquid crystal layer at the side of the polarizing plate. Further, although the electrodes generally have a stripe-like matrix structure, it may take various forms or patterns.
The birefringent plate used in the present invention is such a type that the phase of light is largely changed when the light is passed from one plane to another plane. The birefringent plate has a twist angle of 160.degree.-300.degree. in the same manner as the ordinary TN liquid crystal cell. The birefringent plate may be a liquid crystal cell itself or may be twist birefringent plates or a lamination of phase difference films (retardation films). Namely, it is composed of a combination of two optical media comprising a liquid crystal display cell and a compensation cell.
In order to prepare the birefringent plate, a transparent plastic film such as polycarbonate is uniaxially stretched with accuracy. A plurality of transparent plastic films are laminated with their optical anisotropic axes shifted from each other whereby a twist birefringent plate is formed.
The twist birefringent plate has the same optical characteristics as such one that a number of layers having an optical anisotropy are laminated in such a manner that the optical anisotropic axes are arranged to be sequentially twisted. Generally, it is so formed that a liquid crystal material having a twist characteristics is interposed between two substrates each having an aligning control power, and then, the polymer material is cured.
Conventionally, there has been known a method for a monochrome display that two liquid crystal cells are interposed between a pair of polarizing plates wherein a second liquid crystal cell is used for compensating a first liquid crystal cell. As the optimum conditions used in this case, the twist angle and .DELTA.n.multidot.d of the second liquid crystal layer are substantially equal to the twist angle and .DELTA.n.multidot.d of the first liquid crystal layer, and the direction of the twist angle of the second liquid crystal layer is opposite to the direction of the twist angle of the first liquid crystal layer. Further, the crossing angle between the orientation of liquid crystal molecules at the front surface of the first liquid crystal layer (at the side of second liquid crystal layer) and the orientation of liquid crystal molecules at the front surface of the second liquid crystal cell (at the side of first liquid crystal layer) is determined to be about 90.degree.. Further, the polarizing axis of the polarizing plate is determined to have a crossing angle of about 45.degree. with respect to the orientation of liquid crystal molecules at the front surface of the first liquid crystal layer or the second liquid crystal layer which is at the side of the polarizing plate and which is adjacent to the polarizing plate.
Under such conditions and when a polarizing plate to be disposed at the side of the reflection layer is omitted, a transparent state having a bright achromatic color can be obtained when an applied voltage is low or no voltage is applied.
However, when the applied voltage is gradually increased, a display of white.fwdarw.yellow.fwdarw.blue.fwdarw.yellow.fwdarw.blue.fwdarw.green can be obtained. Such color change is the same as that obtained in the invention of the conventional technique 8 as described in an example. Namely, it is impossible to effect a color change other than a light color or development of a red color.
In the present invention, however, the crossing angle between the orientation of liquid crystal molecules at the front surface of the first liquid crystal cell (at the side of the second liquid crystal cell) and the orientation of liquid crystal molecules at the front surface of the second liquid crystal cell or twist birefringent plate (at the side of the first liquid crystal cell) is shifted by .+-.10.degree. to .+-.40.degree. from .+-.90.degree. rotated position, preferably, 10.degree. to 30.degree., 15.degree. to 25.degree., more preferably about 20.degree., whereby a transparent state of bright achromatic color can be obtained at a non-voltage application time or a non-selected voltage application time.
More detailed description will be made. Supposing that there is a counterclockwise rotation of the orientation from the first orientation to the second orientation in the liquid crystal layer and there is a clockwise rotation from the optical anisotropic axis in the birefringent plate at the side of the liquid crystal layer to the optical anisotropic axis of the birefringent plate at the side opposite the liquid crystal layer, an angle measured counterclockwisely from the optical anisotropic axis in the plane of the birefringent plate at the side of the liquid crystal layer to the first orientation is 100.degree. to 200.degree.. When an applied voltage is gradually increased, a color display of red, blue and green can be provided. In this case, the achromatic color obtained is bright, and excellent color development of red, blue and green is possible.
Description has been made as to requirement of displaying a monochrome tone by using a conventional compensation cell wherein the crossing angle is shifted. However, the same effect can be provided by changing the twist angle, .DELTA.n.multidot.d and dispersion of wavelength.
A temperature compensation type birefringent plate wherein .DELTA.n.multidot.d is changed depending on temperature may be used. In this case, a reflection type color liquid crystal display apparatus capable of providing easy view to a display can be obtained even when an ambient temperature changes. In this case, .DELTA.n.multidot.d which varies depending on a temperature change should be substantially the same as .DELTA.n.multidot.d of liquid crystal which is also changed depending on a temperature change. Further, it is preferable to use a twist birefringent plate wherein a change of the optical anisotropy depends on wavelengths. By using the twist birefringent plate, a reflection type color liquid crystal display apparatus which further improves color purity can be provided.
In the present invention, a white display (W) exhibits color development in a region of 0.3.ltoreq.X&lt;0.33 and 0.3&lt;Y&lt;0.34 in the chromaticity coordinate; a red display (R) exhibits color development in a region of 0.33.ltoreq.X; a blue display (B) exhibits color development in a region of X.ltoreq.0.31 and Y.ltoreq.0.3, and a green display (G) exhibits color development in a region of X.ltoreq.0.30 and 0.3.ltoreq.Y. In the present invention, color development in respective color regions can be effected. Although it is possible to slightly shift the borders of the color regions as long as colors developed in each of the color regions exhibit a high color purity in comparison with those which are out of the borders. Model CR-200 by Minolta was used for measurement of colors.
A further definition about good color development is described as follows. It is supposed that a white display (W) means that coordinate is adjacent to (X=0.31, y=0.316) and a reflectance is more than or equal to 40%, a red display (R) means that coordinate is adjacent to (X=0.5, y=0.3) and a reflectance is more than or equal to 25%, a blue display (B) means that coordinate is adjacent to (X=0.15, y=0.1) and a reflectance is less than or equal to 20%, a green display (G) means that coordinate is adjacent to (X=0.2, y=0.4).